Lesson 4: Dating and Depicting California's Geology

California geologic mapThis is the last of our three lessons that introduce the basic geologic concepts we'll be applying throughout the rest of the semester. Our reading this week covers two chapters—3 and 4—that explore how we establish the ages of geologic features and depict their locations, sizes, and shapes on geologic maps and cross-sections. As you can see from the accompanying geologic map of California, many different types of rocks (marked by different colors) and hundreds of faults (dark lines) are exposed across the state. Even at this scale, however, you'll notice that rocks of similar compositions and ages are found together into several distinct areas. You can see that the Central Valley is underlain by young sedimentary rocks (yellow), for example, whereas the High Cascades and Modoc Plateau in the state's northeastern corner are underlain by volcanic rocks (orange). These areas with similar rock types and geologic structures are California's geomorphic provinces (p. 62) and will provide a framework for our study of the state's geology in the weeks ahead.

In chapter 3 Harden explains how geologists use field observations to determine the relative ages of rock units, and measurements of parent and daughter isotopes to determine their radiometric ages. She also describes how changes in life on Earth have enabled us to establish the geologic time scale, and introduces some of the organisms whose fossils tell us how California's enviroment has changed during the past several hundred million years. In chapter 4 she explains how geologic maps, cross-sections, and block diagrams are used to depict the ages, locations, and orientations of the rock formations that define state's geology. With basic understandings of these topics, as well as plate tectonics and earth materials, you'll be ready to start learning about California's individual geomorphic provinces next week—beginning with young volcanoes here in the High Cascades.

As you read about geologic dating and map interpretation in our text and online it will be important to take detailed notes. A lot of important concepts are covered in these chapters, and writing them out in your own words or making neatly labeled drawings will help you better understand key points and also recognize any gaps in your knowledge. Having good notes will also make it easier for you to review for this week's quiz and and access what you've learned when you need it for future assignments. Be sure that you are prepared to meet the learning objectives outlined below before you move on to the quiz at the bottom of the page.

Weekly Learning Objectives

Upon successful completion of this week's lesson, a student is expected to be able to:

Reading and Browsing Assignment

Exercise 4: Rb-Sr Isochron Dating (Due by 9:00 AM on 7-Feb-2011)

In Chapter 3 you learned a bit about how the decay of radioactive isotopes in minerals or organic materials can be used to determine absolute (numerical) ages of geologic events (p. 44-45). This week's exercise will help you learn more about how radiometric dating works. This is a very important technique because it enables us to reconstruct California's geologic history and estimate the rates of geologic processes operating here.

Quiz 4: Geologic Dating, Maps, and Cross-sections (Due by 9:00 AM on 7-Feb-2011.)

After you feel you have met the learning outcomes outlined above, please complete Quiz 4 in the Etudes "Assignments, Tests, and Surveys" tool. There are ten questions about geologic dating, maps, and cross-sections, each worth one point. If you can answer all of them correctly it means that you know your way around the basics of the techniques we use to determine the ages and locations of rock units in California pretty well and are ready to start learning about volcanism in California next week. Like all of our weekly quizzes, this one is timed (you'll have 30 minutes) and must be completed in one "sitting". (That is, you will only be granted access once.) So, be sure you're ready to complete your quiz when you start it—and be sure you're using Firefox. Good luck.